Trademarks are hereinafter shown in upper case.
The term “potassium monopersulfate” is commonly used in the trade, and is used herein to refer to the mixed triple salt 2KHSO5.KHSO4.K2SO4. The term “KHSO5”, the formula of the active component in the triple salt, is used herein specifically to denote potassium hydrogen peroxymonosulfate.
Potassium monopersulfate as a solid has a theoretical active oxygen content of 5.2%, but as the commercially available solid mixed triple salt, has a typical active oxygen content of about 4.7%. It is recognized that the purity of commercially available potassium monopersulfate is about 95% to about 98% due to the presence of minor impurities, minor amounts of additives (such as an anti-caking agent), and variations in manufacturing processes. “Active oxygen” (AO) is defined as the amount of oxygen in the potassium monopersulfate triple salt in excess of that required to form the corresponding bisulfate. It may be calculated as a percentage from the equation for the decomposition of the monopersulfate,
            KHSO      5        →                  KHSO        4            +              [        O        ]                                Percent            ⁢                          ⁢      AO        =                            Weight                ⁢                                  ⁢                              of                    ⁢                                          [          O          ]                ×        100                              Weight                ⁢                                  ⁢                  of                ⁢                                  ⁢                  KHSO          5                    wherein AO represents the active oxygen and [O] is the oxygen liberated by the decomposition shown. In applying the formula given, the weight of KHSO5 is replaced by the weight of sample where impure material is used. Active oxygen can, of course, be determined from many reactions, the displacement of iodine from potassium iodide, for example, being quantitative. The active oxygen in potassium monopersulfate is available for production of molecular oxygen or for use in various oxidation reactions.
Potassium monopersulfate has a number of uses, including in the microetching of printed wiring boards, also known as printed circuit boards. Tufano, et al., in U.S. Pat. No. 6,818,142 describe a composition and a microetching process using potassium monopersulfate microetching solutions having low inert sulfate concentrations and high weight percent active oxygen. The composition comprised a solution of potassium monopersulfate having a minimum active oxygen content, at an equivalent temperature, equal to from about 1.5 to about 5.5 times the active oxygen content of the triple salt potassium monopersulfate when the potassium monopersulfate is completely dissolved. This is an active oxygen content of 0.7% to 3.36% for the Tufano et al. solutions. The process for the preparation of the solution comprised mixing solid potassium monopersulfate with a quantity of water insufficient to completely dissolve the solid, mixing to form an equilibrated slurry, and separating undissolved solids to obtain a solution. However, it is desired to have potassium monopersulfate solutions having even higher active oxygen content. Further, the storage stability of the Tufano et al. solutions has not been maximized. The insolubility of inert solids that precipitate during storage or shipping remains a problem.
It is desirable to have stable solutions of potassium monopersulfate that have a higher active oxygen content for various uses. It is also desirable to improve the process for microetching of printed wiring boards through use of concentrated aqueous microetching solutions providing lower inert sulfate concentrations and higher weight percent active oxygen. It is also desirable to eliminate the insolubility of components that precipitate as inert solids during storage and/or shipping of potassium monopersulfate solutions. In all applications, the availability of higher active oxygen solutions is more desirable along with minimized inert sulfate salts. The present invention provides such solutions.